Sarcoptes scabiei is a parasitic mite that
lives within the subcutaneous tissues of skin on humans, causing the condition known as
scabies; similar mites cause what is called "mange" in wild and domestic
animals. This mite is distributed worldwide, and can affect all socioeconomic groups.
Scabies mites are generally host specific and S. scabiei is dependant on humans for
its life cycle. These oval, straw coloured mites are very small, measuring 0.2-0.4mm in
length. Their bodies are covered with fine lines and several long hairs. The female mite
has scattered on the dorsal surface some short blunt spines, which aid her in
maintaining her position within the tunnel. The mites have no eyes, and they have short
and thick legs, with the first two pair of legs stalked. The immature stages of the
scabies mite are comprised of a six legged larval stage, followed by 2 nymphal stages that
have eight legs, and each stage resembles the adult mite.

The entire life cycle of the mite occurs over 10-17
days. Newly mated females take approx. an hour to burrow into the outer layer of human
skin and excavates a tunnel. The mite lays her eggs singly, depositing behind her 2-3 eggs
each day. Females burrow without direction, using their mouthparts to tunnel 0.5-5mm a
day, eating the skin and tissue fluids that ooze from their excavations. Each tunnel
contains only one female, her eggs and faeces. After 48 hours the eggs hatch and the
larval stages dig their way to the surface of the skin, where they immediately burrow.
This burrow may only be a short distance into the skin, or they make use of hair
follicles, to moult to the next stage. Larval and nymphal stages remain in these moulting
pockets feeding on fluids secreted from the follicles before moulting to the adult stage.
Newly moulted male and female mites construct short burrows <1mm before mating.

After fertilisation, female mites wander on the skin
to seek a suitable site for a permanent burrow, the transfer of a female to another host
at this stage will initiate a new infection. A fertilised female mite can only initiate
successful scabies infections. Female mites rarely leave their burrows, and if removed by
scratching and remain undamaged, they will attempt to burrow again. During an infection
the number of mites increases rapidly, then drops off, leaving infected persons with a
relatively stable mite population of 15-20 females. The mortality rate of mites is high,
90% of mites that hatch will die, and mites removed from their host can only live a short
time.

Clinical
Presentation

Scabies infestations can present different clinical
pictures and may be difficult to diagnose. The initial infestation may remain undetected
for a month or more, before sensitisation develops and a immunological response in the
host is triggered. The allergic reaction is from components of the mite's faeces, skin
moults, saliva or moulting fluids diffusing into the tissues of the host from the burrows.
The patient can experience severe itching all over the body, and especially at night.
Large areas of the body can be covered by a rash that can last for weeks but which will
not (or only rarely) coincide with the areas of mite infestation. Eruption of the skin
into small itchy lesions may occur in conjunction with the rash. Scabies mites tend to
burrow into the skin where there is a natural crease and the host's reaction will be
minimal. The hands, and webbing between the fingers, the wrists, and elbows are common
areas. From the surface of the skin, the tunnels appear as greyish pencil marks, in darker
skin the tunnels appear paler. The severe itching and scratching can lead to secondary
infections and, in cases of heavy infestations anaemia can develop. There has been no
transmission of disease pathogens associated with this mite.

Untreated scabies infestations, especially in
infants, immobilised geriatric patients, AIDS and other immunologically compromised
patients can support huge numbers of female mites. The patient's skin may become crusted
on the surface, with the underlying layers soft and honeycombed with tunnels, these
infections are referred to as "Norwegian" or "crusted"scabies.
Patients with this advanced state of infection can act as a source for local epidemics in
health care facilities. In some cases, scabies infections in nursing staff or family that
have had contact with the patient will lead to diagnosis of the primary patient.
Reinfected patients will develop an immediate itch when another scabies infection is
initiated.

Laboratory
Diagnosis

Skin scrapings are examined with a compound light
microscope for the presence of mites, eggs or faeces. A glass slide mount is prepared,
using dilute potassium hydroxide or lactic acid to mix with the skin scraping. This aids
in clearing any thick layers of skin cells in the sample to reveal any evidence of the
scabies mite, but clearing may take some time (hours to days).

Treatment &
Control

Once diagnosed, most scabies infections are easy to
control, providing the directions of the scabicide treatments are followed. Any pharmacy
will supply a chemical preparation, and a prescription is not necessary. Care should be
taken in re-applying scabicides unnecessarily, to avoid skin irritation and added costs.
In most cases, itching may persist for a week or more after the treatment, but this is not
necessarily a sign of treatment failure. Re-examination of the patient at four weeks after
the treatment is appropriate. At the commencement of the treatment, bed linen and
underwear of the patient should be washed in hot water and hot tumble dried, but there is
no need to treat furniture or rooms with an insecticide. A common problem of treatment
failure is insufficient coverage of the body with the scabicide, and resistance is rare.
For patients diagnosed with crusted scabies, the patient should be isolated, and barrier
nursing implemented throughout the treatment. All individuals that have had significant
contact with the primary patient should also be treated. Scabies is highly contagious in
overcrowded situations and close contact with infected individuals should be avoided.
Touching, shaking hands, or sharing beds and contaminated objects of an infected person
are common modes of transmission.

Confirmation
& Enquiries

Information and identification of Scabies mites, and all other medically important
arthropods, is provided through the Medical Entomology Department at ICPMR, Westmead
Hospital.